The present invention relates to a method and apparatus for testing fluid containing systems for leakage or diffusion
It is desirable to test the integrity of systems which are designed to retain fluids. For example, in the making of pneumatic tires, if the innerliner does not completely cover the inside surface of the carcass, or if the compound used to make the innerliner does not have the proper air permeability, or if the bead area of the tire has a defect or deformity, the tire may not be capable of retaining air for a suitable period of time. Accordingly, to assure product quality, a specific number of tires from a tire build are subjected to an air retention test. Conventional air retention testing, however, takes sixty or more days to complete. Therefore, if excessive air loss rates exist in a specific tire build, this may not be discovered for two months or more, and a problem may be repeated in subsequent tire builds. There is a need in the art to accelerate tire testing, and the testing of all such systems that depend on the containment of a fluid for its use.
In 1828 Thomas Graham performed experiments studying the rates at which different gases flowed through a small orifice from a container into a vacuum. This phenomenon is called effusion. He discovered that the rate of effusion of a gas varied inversely as the square root of its relative density. The ratio of the relative densities of two gases measured under the same condition is equal to the ratio of their molecular weights. This is expressed as:                                           rate            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            effusion            ⁢                          xe2x80x83                        ⁢            A                                rate            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            effusion            ⁢                          xe2x80x83                        ⁢            B                          =                              (                                          M                ⁢                                  xe2x80x83                                ⁢                                  W                  B                                                            M                ⁢                                  xe2x80x83                                ⁢                                  W                  A                                                      )                                1            /            2                                              (        1        )            
More recently, the American Vacuum Society has outlined AVS standards in defining and testing of mass spectrometer leak detectors. In these vacuum standards they define a leak as xe2x80x9ca hole, porosity, permeable element, or other structure in the wall of an enclosure capable of passing gas from one side of the wall to the other under the action of pressure or a concentration difference existing across the wall.xe2x80x9d The AVS also defines a molecular leak as xe2x80x9ca leak through which the mass rate of flow is substantially proportional to the reciprocal of the square root of the molecular weight of the flowing gas.xe2x80x9d If a leak is molecular, then the AVS recommends that the xe2x80x9cequivalent standard air leak ratexe2x80x9d (air: mol. wt. 29) be taken as (4/29)xc2xd=0.37 times the measured helium (mol. wt 4) leak rate. In other words, helium will leak 2.7 times faster than air in a molecular leak. It is interesting to note that the leak rate is dependent on molecular weight rather than molecular size, as helium, nitrogen, and oxygen have van der Walls radii of 1.33, 1.57, and 1.47 angstroms, respectively. Because helium is much lighter than either nitrogen or oxygen, the speed of the helium atom is much greater than the speed of the oxygen and nitrogen molecule in atmospheric conditions.
It is an object of this invention to provide a method and apparatus whereby fluid retention in pressurized systems can be tested rapidly.
Other objects of the invention will be apparent from the following description and claims.
A method of accelerating testing of a fluid containing system for leaks comprises the steps of (a) providing a sealable container having an internal volume and a shape approximating the volume and shape of a system to be tested for leaks, where the sealable container has a volume of 105% to 1600% of the total volume of the system, (b) filling the system with a known amount of a first fluid having a molecular size or weight less than the molecular size or weight of a second fluid which is normally contained within the system, or a viscosity less than the viscosity of the second fluid, (c) placing the system within the internal volume of the sealable container and sealing the container, (d) creating a pressure differential between the system and the internal volume of the sealable container, wherein fluid used to pressurize the container is different from the fluid used to pressurize the system, (e) monitoring the interior of the container for the presence of the first fluid.
In an illustrated embodiment, the method further comprises the steps of determining the rate of diffusion of the first fluid from the system to the internal volume of the container; and calculating the rate of diffusion of the second fluid from the container based on a factor (f) which describes the difference in diffusion rate of the two fluids in the system.
When the fluid is a gas, the rate of diffusion is calculated using the formula       b    *    =            (                        ⅆ          C                /                  ⅆ          t                    )        ⁡          [                                    (                                          V                c                            -                              V                E                                      )                    ⁢          RT                          PfV          I                    ]      
wherein
b* is the predicted rate of loss of said second fluid by the system;
dC/dt is the test fluid concentration build-up rate in the container
VC is the volume of the container;
VE is the exterior volume of the test system;
R is the gas constant;
T is temperature in Kelvin;
P is absolute pressure of test fluid in the system;
VI is the internal volume of the system; and
f is the factor by which test fluid is leaking at a different rate than system fluid (a ratio) where the rate of the test gas is the numerator.
In the illustrated embodiment, the system is a tire and the system fluid is air, and the method comprises the further steps of selecting the first fluid to be helium, and detecting and monitoring the presence of helium in the internal volume of the container using a mass spectrometer. The presence of the first fluid in the internal volume of the container is monitored and the rate of diffusion is calculated using a Fortran program.
Also provided is an apparatus for measuring the leakage rate of a system comprising a sealable container made of diffusion resistant material and comprising a size and shape suitable for enclosing the size and shape of a system to be tested, a monitoring device adapted to detect the presence and quantity of specific molecules associated with the container, and data collection means for collecting data and calculating the leakage rate of interest.
In the illustrated embodiment, the monitoring device is a mass spectrometer, and the data is collected using a personal computer.